a. Field of Invention
This invention relates to coal liquefaction processes and, in particular, to the use of coal-derived liquids to liquefy coal.
The U.S. Government has retained at least an irrevocable, non-exclusive, paid-up license to make, use and sell the above-identified invention throughout the world by or on behalf of the U.S. Government (including any Government agency) under the provisions of 41 CFR 9-9.109-6.
b. Brief Description of the Prior Art
Because of the decreasing supply and rising price of petroleum, efforts to produce liquid fuels from solid organic materials such as coal have intensified in recent years. Further, environmental concerns have created demand for fuels with a low content of sulfur and nitrogen.
Many direct coal liquefaction processes have been developed. These processes transfer hydrogen to the coal via distillable hydrogen donor or hydrogen shuttler solvents in a coal liquefaction reactor or dissolver. State-of-the-art technology allows production of distillable coal-derived solvents in one of the following ways:
1. thermal hydrogenation of the distillable solvent-coal slurry mixture at elevated temperature and pressure,
2. catalytic hydrogenation of the distillable solvent-coal slurry at elevated temperature and pressure, or
3. catalytic hydrogenation of the distillable solvent followed by thermal hydrogenation of distillable solvent-coal slurry mixture.
Distillable solvents produced by any of these methods are recycled to the inlet of the dissolver where they are mixed with fresh coal. Coal liquefaction processes using distillable solvents frequently produce large quantities of nondistillable liquids undesirable for use as liquid fuels.
In an effort to reduce the yields of nondistillable liquids from coal, many processes have evolved in which certain portions of the product stream from the dissolver are recycled for use as all or part of the solvent in the solvent-coal slurry. Heretofore it has been thought that the distillable fraction of the recycle stream was desirable and functional for solvent purposes. Accordingly, it was known to subject the distillate fraction of the recycle stream to hydrogenating and/or hydrocracking to enhance its hydrogen content prior to use as the solvent. U.S. Pat. Nos. 4,255,248, 4,264,429, 4,264,430 and 4,300,996 are representative of the state-of-the-art in treating recycle solvent streams. Each of these patents, however, teach the effect of hydrotreating the normally liquid, low molecular weight heptane-soluble fraction of the dissolver effluent prior to use as the recycle solvent.
Others have recognized the importance of recycling a solvent slurry consisting of nondistillable, normally solid dissolved coal and mineral residue. U.S. Pat. Nos. 4,230,556, 4,227,991, 4,222,848, 4,222,847, 4,211,631, 4,203,823, 4,189,375, 4,159,238, 4,159,237 and 4,159,236 are representative of this technology. Each of these patents emphasizes the catalytic effects of the mineral residue in the recycle slurry. However, these patents do not mention solvent upgrading. Heretofore the importance of upgrading high molecular weight, normally solid dissolved coal that is essentially mineral residue free in a hydrotreater prior to use as a recycle solvent has gone unrecognized. Normally solid essentially mineral residue free dissolved coal is substantially pyridine (m-cresol, tetrahydrofuran) soluble vacuum still bottoms which boil above about 800.degree. F. to about 1000.degree. F. and will hereinafter be referred to as "bottoms".
According to the present invention it is possible to maximize or minimize production of light liquid products from coal. Higher yields of liquid products boiling in the range of about C.sub.4 -800.degree. F. are obtained from this process than from other state-of-the-art methods of direct coal liquefaction. The present invention is based upon the recognition of the importance of the bottoms, defined above, to which limited amounts of hydrogen have been added by mild hydrotreatment, in the solvent recycle stream.